Question

What is the most common fissionable isotope in a commercial nuclear power reactor?

Short answer

The most common fissionable isotope used in commercial nuclear power reactors is uranium-235 (\(^{235}\text{U}\)).

Step-by-step solution

Identifying the most common fissionable isotope in commercial nuclear power reactors

The two most common types of commercial nuclear power reactors are Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs). Both of these reactors use nuclear fuel in the form of pellets made from enriched uranium. Enriched uranium contains a higher percentage of the desired fissionable isotope, uranium-235 (\(^{235}\text{U}\)), compared to natural uranium.

Importance of Uranium-235

The fissionable isotope uranium-235 is used because it has a higher probability of undergoing fission when it captures a neutron compared to other isotopes, such as uranium-238 (\(^{238}\text{U}\)). Therefore, uranium-235 releases more energy during nuclear reactions and makes it the primary choice for nuclear power production.

Conclusion

The most common fissionable isotope used in commercial nuclear power reactors is uranium-235 (\(^{235}\text{U}\)).

Key concepts

Uranium-235

Uranium-235 (denoted as \(^{235}\text{U}\)) is an isotope of uranium which plays a crucial role in the production of nuclear energy. It is one of the few materials that can sustain a nuclear chain reaction, making it essential for nuclear reactors and weapons. Given its status as a fissionable isotope, it possesses a high likelihood of undergoing fission when it absorbs a neutron.
When a uranium-235 nucleus absorbs a neutron, it becomes unstable and splits into two smaller nuclei along with some additional neutrons and a significant amount of energy. This release of energy is harnessed in nuclear power plants to produce electricity. Importantly, the additional neutrons emitted can strike other uranium-235 nuclei, continuing the chain reaction.
Though it constitutes only about 0.7% of natural uranium, its efficiency in fuel use makes it highly valuable. Its ability to fission with low-energy (thermal) neutrons, unlike other isotopes like uranium-238, optimizes its use in nuclear reactors, thereby making uranium-235 a preferred choice for energy production.

Nuclear Power Reactors

Nuclear power reactors are facilities that generate electricity through nuclear reactions—primarily fission. The two most common types of reactors in commercial use are Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs). Both types rely heavily on uranium-235 as fuel due to its ability to sustain a continuous nuclear chain reaction.
PWRs and BWRs function by using uranium fuel, often in the form of small ceramic pellets, which are placed in metal tubes or "fuel rods." These fuel rods form bundles that are submerged in water, which acts as a coolant and, in the case of BWRs, also directly generates steam for turbines. Through fission, uranium-235 generates immense heat, causing water to turn into steam that spins turbines connected to electrical generators.

• Pressurized Water Reactors (PWRs): In these reactors, water used as a coolant is kept at high pressure to prevent it from boiling. The heat generated from the fission process is transferred to a secondary loop where it boils a separate water supply to produce steam.
• Boiling Water Reactors (BWRs): Boil water directly in the reactor vessel to form steam, which then drives the turbines.

By mastering control over the fission process, reactors provide a steady and reliable supply of electricity without emitting greenhouse gases during operation.

Enriched Uranium

Enriched uranium refers to uranium that has undergone a process to increase the concentration of \(^{235}\text{U}\) isotopes relative to \(^{238}\text{U}\) isotopes. Natural uranium contains about 0.7% \(^{235}\text{U}\) and 99.3% \(^{238}\text{U}\). However, for nuclear reactors to operate efficiently, a higher concentration of \(^{235}\text{U}\) is needed—typically around 3-5%.
The enrichment process entails separating isotopes based on their slight differences in mass. There are several methods for enrichment, with the most common ones being gas centrifugation and gaseous diffusion. Enriched uranium ensures a higher probability of fission and a more consistent rate of energy production.
Aside from its critical use in civilian nuclear power reactors, enriched uranium also has crucial implications for nuclear weapons, where concentrations of \(^{235}\text{U}\) must be much higher (often over 90%) to achieve a sufficient explosion.
Providing a robust and efficient fuel source, enriched uranium addresses energy demands, although it carries proliferation risks if not strictly controlled in international agreements and safeguards.